Base regen (which is an equation that factors in both Intellect and Spirit) will be nerfed by about 40%. This means the mana you would regen while just standing around doing nothing will be quite a bit lower than it currently is. However, in exchange, regen-while-casting talents such as Meditation, Intensity, Spirit Tap and many others, whille be buffed so your in-combat mana regen shouldn't be harmed very much. It'll essentially prevent you from trying to wiggle in and out of the 5 second rule for big regen ticks, while not terribly bothering your mana regen while you're actually doing things.
Since Paladins are hardly affected by these changes at all, they're getting hit in a different way. We have the Divine Plea change which brought the healing debuff to 50%, and they'll be nerfing Spiritual Attunement for all but Protection Paladins. Hopefully that doesn't hit our Retribution friends too badly, but we will see.
Ghostcrawler has a lot of other details in the thread (which you should read through, at least his posts), but I'm pretty skeptical. I'm not so sure these changes will really fix much, which means that they'll need to look for other ways to make Mana Regen something you worry about. I'm admittedly pessimistic since I have a Shadow Priest, but I have no doubt that Replenishment will need to be nerfed again. There's such a massive difference in mana regen between when you have it and when you don't that I'm pretty sure it'll get brought down again in the future. Sure, it's supposed to be as important as a tank, but if your Tank was completely impenetrable that would be nerfed, too.
We'll see what happens, hm?